1. Field of the Invention
The present invention relates generally to transmissions having multiple lay shaft branches for driving a common output gear and more specifically to a system and method for improved operation when clutches are employed in the branches.
2. Brief Description of the Prior Art
Transmissions having an input shaft which drives multiple lay shaft branches with each branch, in turn, including a pinion driving a common output gear have been frequently employed for the purpose of distributing a load through the plurality of gear trains. Typical of such gearing systems is the gearing system disclosed in U.S. Pat. No. 2,386,367.
Transmissions of this type have been employed to drive bowl mill mixers with the input shaft including a double helical pinion. The double helical pinion engaged a single helical gear on each lay shaft and the lay shaft pinions engaged an output gear. If the helical portions of the double helical input pinion had opposite slope and the same lead and if the double helical input pinion was permitted to move axially, the load distribution was equally divided between each of the helical portions of the input pinion and each lay shaft. In such operating state, the axial position of the input pinion was determined by the engagement with the driven gears of the lay shafts. The lay shafts were synchronized by the engagement of the lay shaft pinions with the output gear.
In some applications, such as in marine systems, it was necessary to provide shiftable clutches within the lay shaft branches between the gears driven by the input pinion and the pinions engaging the output gear. Shiftable clutches were also necessary to obtain reversibility of the direction of rotation of the output gear. In such instances, it was necessary to provide additional lay shafts so that some lay shafts were rotating in opposite directions. Shiftable clutches were employed in a disengaged position in the lay shafts which rotated in the direction opposite to that of the lay shafts driving the output gear. A typical example of a transmission gearing having lay shaft branches and reversing capabilities is illustrated in U.S. Pat. No. 1,151,762.
Unfortunately, when shiftable clutches were employed in the lay shaft branches, the advantages of equal load distribution between the branches as a result of an axially freely movable opposite double helical drive pinion were forfeited. Examples of transmissions employing shiftable clutches were illustrated and described in the journal MTZ Motortechnische Zietschrift 38 (1977) 11 at page 505 (FIGS. 8 and 9) which disclosed reversing gear systems for ships.
In one system shown (FIG. 8), herringbone spur gears were employed on the lay shafts in engagement with a herringbone drive pinion. Thus, the drive pinion was not axially movable. As a result, when the clutches were in engagement, each of the lay shaft branches bore different loadings due to the different frictional characteristics of each clutch.
In a second ship reversing gear system described in the journal (FIG. 9), an input drive pinion with single helical teeth was illustrated. The drive pinion engaged two spur gears, each fixed to a lay shaft. The axial forces due to engagement of the helical teeth were restrained by thrust bearings. The system did not permit equal torque distribution through axial movement of the input pinion. Unequal torque distribution over the two lay shafts resulted.
The problems encountered with the employment of frictional clutches or slip clutches resulted because at the end of the slip phase of each clutch the clutch reached its synchronous state at a torque which was dependent upon its frictional characteristics. The load distribution through each lay shaft branch thus depended upon the frictional characteristics of the clutch. With the input pinion axially fixed and the transmission operating at full speed with the frictional clutches engaged, different load distributions and different load tensions resulted in each of the lay shafts.
A similar transmission system employing form locking clutches such as gear or claw clutches was illustrated in the journal MTZ Motortechnische Zietschrift 38 (1977) 11 at page 506 (FIGS. 11 and 12). Normally, the clutching operation with gear or jaw clutches could only be performed when the input shaft was stopped or rotated very slowly since position synchronism between the primary and secondary parts of such clutches was a prerequisite to engagement.